Image processing apparatus and image processing method

ABSTRACT

In image processing, it is possible to suppress density fluctuation and keep graininess low as well as obtain a good balance of the processing load. More specifically, when dividing multi-valued data and generating two-pass multi-pass printing data, divided multi-valued data that is common to the two passes is generated in addition to the divided multi-valued data for each of the two passes. Moreover, quantized data of that common multi-valued data is reflected on the quantized data for each pass. Furthermore, when generating quantized data, a process of generating common data by the aforementioned data division, or a process of performing quantization first without dividing the multi-valued data and then dividing the quantized 2-pass data is selectively performed according to the printing position on printing medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus and imageprocessing method, and more particularly to a technique for generatingdata for completing printing in the same area of a printing medium bymoving a printing head a plurality of times relative to the same area ofthe printing medium.

2. Description of the Related Art

As this kind of technique, a multi-pass printing method is known forcompleting an image to be printed in the same area of a printing mediumby scanning that same area of the printing medium a plurality of timeswith a printing head. With this technique, it is possible to reducedensity unevenness and stripes in an image. However, even though themulti-pass printing method is employed, there is a possibility that ashift of the dot printing position will occur between the pluralityprinting scans due to fluctuation in the conveyance amount of theprinting medium. Such shift causes fluctuation in the dot coverage, andas a result, image defects such as density fluctuation or densityunevenness may occur.

A method is known as a technique for reducing the image defects causedby the shift of the dot printing position due to the above describedfluctuation of the conveyance amount in which image data are dividedinto respective data corresponding to different scans in the stage ofmulti-valued image data before binarization, and then the respectivedivided multi-valued image data are independently binarized (JapanesePatent Laid-Open No. 2000-103088). FIG. 13A is a diagram illustratingthe arrangement of dots that are printed based on image data that areprocessed by the method disclosed in Japanese Patent Laid-Open No.2000-103088. In the figure, the black dots 551 are dots that are printedin a first printing scan, white dots 552 are dots that are printed in asecond printing scan and gray dots 553 are dots that are printed byoverlapping of dots that are printed in the first printing scan andsecond printing scan.

With this dot arrangement, even though a dot group printed in a firstprinting scan and a dot group printed in a second printing scan becomeshifted in the main scanning direction or sub scanning direction, thereis not much fluctuation in the dot coverage on the printing medium, andas a result, it is possible to reduce the image defects mentioned above.This is because even though portions, where dots that are printed in thefirst printing scan and dots that are printed in the second printingscan overlap, newly appear, there are portions where two dots that wereoriginally supposed to be printed such that they overlap do not overlap.More specifically, conventionally, by using a mask on normally quantizeddata, print data are divided, as well as complementarity and exclusivityis given to data that are to be printed in different printing scans. Onthe other hand, the method disclosed in Japanese Patent Laid-Open No.2000-103088 is a method in which multi-valued data are divided in themulti-valued data stage into a plurality of multi-valued data thatcorrespond to different printing scans, and the plurality ofmulti-valued data are each independently quantized so that quantizeddata to be used in the respective printing scans are obtained. Thisallows complementarity between dots that are printed in differentprinting scans to be decreased, and overlapping dots are caused to occuramong the dots that are printed in a plurality of printing scans.

However, in the method disclosed in Japanese Patent Laid-Open No.2000-103088 it is not possible to control the overlap amount of dotsthat are printed in a plurality of printing scans. As a result, thenumber of overlapping dots may become excessive and the graininess ofthe image may be increased, or conversely, the number of overlappingdots could be too few and the aforementioned density fluctuation may notbe sufficiently suppressed.

The inventors of the present application placing attention on making itpossible to control the amount of the dot overlap by generating datathat is reflected in all the quantized data of the different printingscans in common with each other.

On the other hand, the fluctuation in the conveyance amount of theprinting medium, which causes the density unevenness mentioned above,differs according to the position of the printing medium on theconveying path of the printing medium. For example, when printing overthe entire surface of a printing medium without setting margins, or inother words, performing so-called margin-less printing, the printingposition is located on the printing medium where printing is performedby the printing head as the printing medium is conveyed along theconveying path by only one pair of conveying rollers that are providedon both the upstream side and downstream side of the printing area. Insuch a printing position, the conveyance precision is low and it is easyfor fluctuation in the conveyance amount to occur. On the other hand,when the printing medium is conveyed in a state of being held by both ofa pair of conveying rollers on the upstream side and downstream side,the conveyance precision is high and it is difficult for fluctuation ofthe conveyance amount to occur. Moreover, it is known that duringconveyance, when the printing medium enters or leaves the nip portion ofa pair of conveying rollers, there is a relatively large fluctuation inthe conveyance amount.

When there is various fluctuation in the amount of conveyance such asthis, applying a process of generating data that is commonly reflectedin all of the data may perform the processing described above forgenerating data for not so necessary printing positions and thus causesan increase in processing load or decrease in processing speed. Forexample, the process of dividing multi-valued data and quantizing theindividual multi-valued data brings about an increase in the processingload.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an image processingapparatus and image processing method capable of suppressing densityfluctuation as mentioned above and keeping graininess low, as well askeeping a good balance of the processing load.

In a first aspect of the present invention, there is provided an imageprocessing apparatus that, for printing an image on a pixel area of aprinting medium at respective printing positions on the printing mediumby performing a plurality of relative scans of a printing head withrespect to the printing medium that is conveyed to differentiate theprinting positions, processes multi-valued image data that correspondsto the image to be printed on the pixel area, the apparatus comprising:a selecting unit configured to select one of a first process and asecond process that process the multi-valued image data corresponding tothe printing position in accordance with the printing position on theprinting medium; in the case that the selecting unit selects the firstprocess, a first division unit configured to divide the multi-valuedimage data into respective multi-valued data that correspond to theplurality of relative scans and multi-valued data that correspondscommonly to at least two relative scans among the plurality of relativescans; a first quantization unit configured to perform quantizationprocessing for each of the multi-valued data divided by the firstdivision unit to generate respective quantized data that correspond toeach of the plurality of relative scans and quantized data thatcorresponds commonly to the at least two relative scans; and acombination unit configured to combine the quantized data generated bythe first quantization unit for each corresponding relative scan togenerate combined quantized data that corresponds to each of the atleast two relative scans; or in the case that the selecting unit selectsthe second process, a second quantization unit configured to performquantization processing for the multi-valued image data to generatequantized data; and a second division unit configured to divide thequantized data generated by the second quantization unit into respectivequantized data that correspond to each of the plurality of relativescans.

In a second aspect of the present invention, there is provided an imageprocessing method of, for printing an image on a pixel area of aprinting medium at respective printing positions on the printing mediumby performing a plurality of relative scans of a printing head withrespect to the printing medium that is conveyed to differentiate theprinting positions, processing multi-valued image data that correspondsto the image to be printed on the pixel area, the method comprising: adetection step of detecting the printing position on the printingmedium; and a selecting step of selecting one of a first process and asecond process that process the multi-valued image data corresponding tothe printing position in accordance with the printing position detectedby the detection step, wherein the first process including: a firstdivision step of dividing the multi-valued image data into respectivemulti-valued data that correspond to the plurality of relative scans andmulti-valued data that corresponds commonly to at least two relativescans among the plurality of relative scans; a first quantization stepof performing quantization processing for each of the multi-valued datadivided by the first division unit to generate respective quantized datathat correspond to each of the plurality of relative scans and quantizeddata that corresponds commonly to the at least two relative scans amongthe plurality of relative scans; and a combination step of combining thequantized data generated by the first quantization unit for eachcorresponding relative scan to generate combined quantized data thatcorresponds to each of the at least two relative scans; and the secondprocess including: a second quantization step of performing quantizationprocessing for the multi-valued image data to generate quantized data;and a second division step of dividing the quantized data generated bythe second quantization unit into respective quantized data thatcorrespond to each of the plurality of relative scans.

With the structure described above, multi-value image data of an imagethat corresponds to a printing position on a conveyed printing medium isdivided at division ratios according to that printing position. Thereby,when printing at a printing position having low conveyance precision, itis possible to control the amount of dot overlap by performing a processof dividing that multi-value image data in which division ratios areadequately determined and then quantizing the divided data. On the otherhand, for example, when the printing position is such that there is noneed dividing the multi-value image data and then quantizing thatdivided data is not necessary, it is possible to make the divisionratios zero. As a result, it is possible to balance suppressing densityfluctuation with keeping graininess low and the processing load.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are views explaining the construction of a printer thatfunctions as the image processing apparatus of the present invention;

FIG. 2 is a diagram explaining two-pass multi-pass printing;

FIG. 3 is a block diagram illustrating the construction of the main partrelated to control of the printer in FIG. 1;

FIG. 4 is a diagram illustrating printing positions on a printing mediumwhen the printing medium is conveyed, in embodiments of the presentinvention;

FIG. 5 is a diagram showing a relationship between FIG. 5A and FIG. 5B,and FIGS. 5A and 5B are block diagrams illustrating construction forperforming a printing data generation process (image processing) fortwo-pass printing of a first embodiment of the present invention;

FIG. 6 is a diagram schematically expressing the image processingillustrated in FIGS. 5A and 5B (image data division process→quantizationprocess→quantized data combination process);

FIGS. 7A to 7C are diagrams expressing an error-distribution matrix thatillustrates error-distribution coefficients for peripheral pixels whenperforming an error diffusion process;

FIG. 8 is a flowchart illustrating the exclusive error-diffusion processfor generating binary data in a first embodiment;

FIG. 9 is a graph showing a relationship between a degree ofapproximation of a distance to boundary of a leading end portion or atrailing end portion, and division ratios for multi-valued data;

FIG. 10 is a diagram showing a relationship between FIG. 10A and FIG.10B, and FIGS. 10A and 10B are flowcharts illustrating the exclusiveerror-diffusion process for generating three-value data in anotherembodiment of the invention;

FIG. 11 is a block diagram illustrating construction for performing aprinting data generation process (image processing) for performing3-pass printing;

FIG. 12 is a block diagram; and

FIGS. 13A and 13B are diagrams illustrating the dot arrangement of dotsprinted in two scans.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described in detail belowwith reference to the drawings. The embodiments explained below relateto an inkjet printing apparatus as an example. However, the presentinvention is not limited to an inkjet printing apparatus. As long as theapparatus uses a method of printing an image on a printing medium by aprinting unit during relative scan between a printing head for printingdots, and the printing medium, the invention can also be applied to theapparatus other than an inkjet printing apparatus.

In this specification, “multi-pass printing” refers to a printing methodin which an image to be printed in the same area of a printing medium iscompleted by a plurality of relative scans (relative movement) between aprinting head and printing medium. “Relative scan (relative movement)between a printing head and printing medium” refers to an operation of aprinting head moving (scanning) relative to a printing medium, or aprinting medium moving (being conveyed) relative to a printing head. Ona micro level, the term “same area” refers to “one pixel area”, and on amacro level refers to an “area that can be printed during one relativescan”. A “pixel area (may simply be called a “pixel”)” refers to thesmallest unit of area for which gradation expression is possible usingmulti-valued image data. On the other hand, an “area that can be printedduring one relative scan” refers to an area on a printing medium overwhich a printing head passes during one relative scan, or is an areathat is smaller than this area (for example, one raster area). Forexample, in a serial printing apparatus, when an M-pass (M is an integer2 or greater) multi-pass mode is executed as illustrated in FIG. 2, on amacro level the same area can be defined as one printing area in thefigure (an area having a nozzle array width of 1/M).

Hereinafter, the “relative scan” will be simply referred to as a “scan”.For example, in the case of three-pass multi-pass printing, relativescan is performed three times (first relative scan, second relativescan, third relative scan) for one pixel area, and these first throughthird relative scan are called a “first scan”, “second scan” and “thirdscan”, respectively.

<Printing Apparatus Construction>

FIG. 1A is a perspective view of a photo-direct printer (hereafterreferred to as a PD printer) 1000 that can be applied as the imageprocessing apparatus of the present invention. The PD printer 1000 has afunction for receiving data from a host computer (PC) to performprinting, a function of directly reading an image that is stored on amemory medium such as a memory card to perform printing, and a functionof receiving an image from a digital camera, PDA or the like to performprinting.

In FIG. 1A, reference numeral 1004 denotes a discharge tray on whichprinted paper can be stacked, and reference numeral 1003 denotes anaccess cover that the user can open or close when replacing the printinghead cartridge or ink tank that are housed inside the main unit. Menuitems for setting various printing conditions (for example, type ofprinting medium, image quality, etc.) are displayed on a operation panel1010 that is provided on the upper case 1002, and the user can set theseitems according to the type and usage of the image to be output.Reference numeral 1007 denotes an auto feeding unit that automaticallyfeeds printing medium to the inside of the printer, reference numeral1009 denotes a card slot into which an adapter, in which a memory cardcan be mounted, is inserted, and reference numeral 1012 denotes a USBterminal for connecting a digital camera. A USB connector for connectinga PC is provided on the rear surface of the PD printer 1000.

FIG. 1B is a perspective view illustrating the internal construction ofthe PD printer. A printing medium P is fed by the auto feeding unit 1007to the nip portion between the conveying roller 5001 that is located onthe conveying path, and a pinch roller 5002 that follows the conveyingroller. After that, the printing medium P is conveyed by the rotation ofthe conveying roller 5001 in the direction of arrow A (sub scanningdirection) in the figure while being guided and supported on a platen5003. The pinch roller 5002 elastically presses toward the conveyingroller 5001 by a pressure unit such as a spring (not shown in thefigure). The conveying roller 5001 and pinch roller 5002 are componentelements of a first conveying unit that is located on the upstream sidein the conveyance direction of the printing medium.

The platen 5003 is located at the printing position and is opposed to asurface of the inkjet type printing head 5004 where the ejection portsare formed, and by supporting the rear surface of the printing medium P,maintains a constant distance between a front surface of the printingmedium P and the ejection port surface. The printing medium P that isconveyed and printed over the platen 5003 is conveyed in the direction Awith being held between a discharge roller 5005 and spurs 5006, whichare rotating bodies that follow the discharge roller 5005, and isdischarged into the discharge tray 1004 from the platen 5003. Thedischarge roller 5005 and spurs 5006 are component elements of a secondconveying unit that is located on the downstream side in the conveyingdirection of the printing medium.

As described above, as construction for conveying the printing mediuminside the printer of the embodiments, a pair of a conveying roller 5001and a pinch roller 5002 are provided on the upstream side of theprinting head, and a pair of a discharge roller 5005 and spurs 5006 areprovided on the downstream side.

The printing head 5004 is mounted on a carriage 5008 such that it isremovable, and such that the ejection port surface faces the platen 5003and printing medium P. The driving force of a carriage motor E0001 movesthe carriage 5008 back and forth along two guide rails 5009 and 5010,and during this movement, the printing head 5004 executes the inkejection operation according to print signals. The direction in whichthe carriage 5008 moves is a direction that crosses the direction(direction of arrow A) in which the printing medium P is conveyed.Printing is performed on the printing medium P by repeating the mainscanning (movement during printing) of the carriage 5008 and printinghead 5004 and conveyance (sub scanning) of the printing medium Palternately.

FIG. 1C is a schematic diagram when viewing the printing head 5004 fromthe surface on which the ejection ports are formed. In the figure,reference numeral 51 denotes a cyan nozzle array, reference numeral 52denotes a magenta nozzle array, reference numeral 53 denotes a yellownozzle array and reference numeral 54 denotes a black nozzle array. Thewidth in the sub scanning direction of each of the nozzle arrays is d,and it is possible to perform printing of a width d during one scan.

Each of the nozzle arrays 51 to 54 are constructed such that there are1200 nozzles arranged in the sub scanning direction at 600 dpi(dots/inch), or in other words at intervals of approximately 42 μm. Eachindividual nozzle comprises an ejection port, an ink path for guidingthe ink to the ejection port, and an electro-thermal conversion elementthat causes film boiling to occur in the ink near the ejection port.With this construction, by applying a voltage pulse to each individualelectro-thermal conversion element according to an ejection signal, filmboiling occurs in the ink near the electro-thermal conversion elements,and an amount of ink that corresponds to the growth of generated bubbleis ejected from the ejection ports as droplets.

<Multi-Pass Printing>

The printing apparatus of the present embodiment is can performmulti-pass printing, and in this printing, an image is formedstep-by-step by a plurality of printing scans to an area in which theprinting head 5004 can print in one time of printing scan. An image isformed step-by-step by performing a conveyance operation of a smallamount that is less than the width d of the printing head 5004 betweensuccessive printing scans and differentiating nozzles used in each step.This allows density unevenness or stripes that occur due to variationsin the ejection characteristics of the nozzles to be reduced. Whether ornot to perform multi-pass printing, or the number of multi-passes(number of printing scans performed for the same area) is suitably setaccording to image information that is input from the operation panel1010 by the user, or that is received from a host device.

Next, FIG. 2 will be used to explain an example of multi-pass printingthat can be executed by the printing apparatus described above. Here,two-pass printing will be explained as an example of multi-passprinting; however, the present invention is not limited to two-passprinting, and the multi-pass printing may also be M (M is an integer 2or greater)-pass printing such as three-pass, four-pass, eight-pass,sixteen-pass printing and the like. In the present invention, the“M-pass mode (M is an integer 2 or greater)” that can be preferablyapplied is a mode in which printing is performed in the same area of aprinting medium by M scans of a printing element group between which theconveyance of the printing medium having a smaller amount than the widthof the array range of printing elements intervenes. In the M-pass mode,the amount of one conveyance of the printing medium is preferably set tobe equal to an amount corresponding to 1/M the width of the array rangeof the printing elements, and by performing this setting, the width inthe conveyance direction of the same area is equal to a width thatcorresponds to the amount of one conveyance of the printing medium.

FIG. 2 is a diagram schematically illustrating the state of two-passprinting, and showing the relative positional relationship between theprinting head 5004 and printing area in the case of printing in a firstprinting area to fourth printing area, which correspond to four sameareas. In FIG. 2, only one nozzle array (printing element group) 51 ofone color of the printing head 5004 shown in FIG. 1C is shown. Inaddition, herein after, of a plurality of nozzles (printing elements)composing the nozzle array (printing element group) 51, the nozzle groupthat is located on the upstream side in the conveyance direction iscalled an upstream side nozzle group 51A, and the nozzle group that islocated on the downstream side in the conveyance direction is called adownstream side nozzle group 51B. Moreover, the width in the subscanning direction (conveying direction) of each of the same areas (eachprinting area) is equal to the width (640 nozzle widths) thatcorresponds approximately to half of the width (1280 nozzle widths) ofthe array range of the plurality of printing elements of the printinghead.

In the first scan, only part of the image to be printed in the firstprinting area is printed using the upstream side nozzle group 51A. Thegradation value of the image data that are printed by this upstream sidenozzle group 51A is reduced to approximately half of the gradation valueof original image data (multi-valued image data that corresponds to theimage to be finally printed in the first printing area), for eachindividual pixel. After the above described printing by the first scanis complete, the printing medium is conveyed along the Y direction byjust the distance corresponding to 640 nozzles.

Next, in a second scan, only part of the image that is to be printed ina second printing area is printed using the upstream side nozzle group51A, and the image to be printed in the first printing area is completedusing the downstream side nozzle group 51B. For the image data that isprinted by this downstream side nozzle group 51B as well, the gradationvalue is reduced to approximately half of the gradation value of theoriginal image data (multi-valued image data that corresponds to theimage to be finally printed in the first printing area). This allows theimage data whose gradation value has been reduced to approximately ½ tobe printed two times in the first printing area, and therefore thegradation value of the original image data is conserved. After the abovedescribed printing by the second scan is completed the printing mediumis conveyed in the Y direction just a distance equal to the amount of640 nozzles.

Next, in a third scan, only part of the image that is to be printed inthe third printing area is printed using the upstream side nozzle group51A, and the image to be printed in the second printing area iscompleted using the downstream side nozzle group 51B. After that, theprinting medium is conveyed in the Y direction just a distancecorresponding to the amount of 640 nozzles. Finally, in a fourth scan,only part of the image that is to be printed in the fourth printing areais printed using the upstream side nozzle group 51A, and the image to beprinted in the third printing area is completed using the downstreamside nozzle group 51B. After that, the printing medium is conveyed inthe Y direction just a distance corresponding to the amount of 640nozzles. The same printing operation is also performed for otherprinting areas. By repeating the above described printing main scan andconveyance operation described above, the two-pass printing is performedfor all of the printing areas.

<Electrical Specifications of the Control Unit>

FIG. 3 is a block diagram illustrating the configuration of the mainpart related to control of the PD printer 1000 shown in FIG. 1A. In FIG.3 the same reference numbers are given to parts that are common to thosein the aforementioned figures, and an explanation of those parts isomitted. As will be clear from the following explanation, the PD printer1000 functions as an image processing apparatus.

In FIG. 3, reference numeral 3000 denotes a control unit (control board)and reference numeral 3001 denotes an image processing ASIC (speciallycustomized LSI). Reference numeral 3002 denotes a DSP (digital signalprocessor), which includes an internal CPU to perform various controlprocessing and image processing shown in FIGS. 5A and 5B and otherfigures. Reference numeral 3003 denotes a memory and the memory 3003includes a program memory 3003 a that stores the control programs forthe CPU of the DSP 3002, a RAM area that stores programs duringexecution, and a memory area that functions as a work memory the storesimage data and the like. Reference numeral 3004 denotes a printerengine, and in this embodiment the printer engine for an inkjet printingapparatus that prints color images using a plurality of colors of colorink is mounted. Reference numeral 3005 denotes a USB connector that isused as a port for connecting a digital camera (DSC) 3012. Referencenumeral 3006 denotes a connector for connecting a viewer 1011. Referencenumeral 3008 denotes a USB hub (USB HUB), and when the PD printer 1000performs printing based on image data from the PC 3010, the USB hubpasses the data from the PC 3010 as is and outputs that data to theprinter engine 3004 via the USB 3021. Thereby, the connected PC 3010 isable to execute printing by directly exchanging data and signals withthe printer engine 3004 (functions as a normal PC printer). Referencenumeral 3009 denotes a power-supply connector, and by way of the powersupply 3019 inputs DC voltage that as been converted from commercial AC.Reference numeral 3010 denotes a typical personal computer, referencenumeral 3011 denotes the aforementioned memory card (PC card), andreference numeral 3012 denotes a digital camera (DSC). The exchange ofsignals between the control unit 3000 and printer engine 3004 isperformed by way of the aforementioned USB 3021 or an IEEE 1284 bus3022.

First Embodiment

A first embodiment of the present invention relates to a data generationmode which when dividing multi-valued data to generate multi-valued datafor the multi-pass printing of two-pass that is described abovereferring to FIG. 2, generates divided multi-valued data that is commonfor two passes as well as divided multi-valued data for each of thetwo-pass. In addition, the data generation mode of this embodimentreflects the quantized data of the common multi-valued data on quantizeddata for each pass. Furthermore, the data generation of the presentinvention selectively performs a process of generating common data bydividing of the multi-valued data or a process of dividing quantizeddata into each of data for two-pass printing without the dividing ofmulti-valued data.

FIG. 9 is a diagram explaining the printing positions on a printingmedium when the printing medium is conveyed, according to thisembodiment. In FIG. 4, a leading end portion (leading end area) of theprinting medium P is the printing position when the printing medium isconveyed in a condition that the printing medium is held between theupstream conveying roller 5001 and pinch roller 5002 (upstream rollerpair). Next, a boundary between the leading end portion and a middleportion (middle area) is the printing position when a leading edge ofthe printing medium is inserted into a nip portion between thedownstream discharge roller 5005 and spurs 5006, when the printingmedium is conveyed. When the printing medium enters the nip portion, theamount of conveyance may become greater than the normal conveyanceamount, and in that case the displacement of the printing positionbecomes comparatively large.

The middle portion is the printing position when the printing medium isconveyed in a condition that the printing medium is held between boththe set of the upstream conveying roller 5001 and pinch roller 5002, andthe set of the downstream discharge roller 5005 and spurs 5006.

Furthermore, a boundary between the middle portion and a trailing endportion (trailing end area) is the printing position when the printingmedium leaves from the nip portion between the upstream conveying roller5001 and the pinch roller 5002. When the printing medium leaves from thenip portion as well, as in the case of entering the nip portion asmentioned above, the conveyance amount may suddenly become greater thannormal, and in that case the displacement of the printing positionbecomes comparatively large. Finally, the trailing end portion is theprinting position when the printing medium is conveyed in a conditionthat the printing medium is held only between the downstream dischargeroller 5005 and spurs 5006 (downstream roller pair).

The printing positions on the printing medium described above can beknown by detecting the position of the printing medium in the conveyancepath. In this embodiment, the position of the printing medium isdetected according to the amount of rotation of the conveying rollerthat is measured from the time that a specified sensor that is providedin the conveyance path detects the leading edge of the printing medium.The amount of rotation can be known by a signal from an encoder that isprovided on the conveying roller. Even in the case in which the printingmedium is conveyed only by the discharge roller, the amount of rotationof the conveying roller that rotates with the discharge roller can beknown. In addition, of the printing positions, the “boundary” mentionedabove can be determined by the encoder to be the printing position wherethe amount of rotation becomes a specified amount greater than theamount of rotation during normal rotation. The system explained abovefor detecting the printing position is used in the generation ofquantized data of this embodiment as will be explained next withreference to FIGS. 5A and 5B.

Of course the system for detecting the printing position of the printingmedium is not limited to the system described above, and any well-knownmethod could be used.

FIGS. 5A and 5B are block diagrams showing a system for performing aprocess (image processing) for generating printing data for the two-passprinting, according to this embodiment. The multi-valued image datainput section 401, color conversion processing section 402, colorseparation processing section 403, gradation correction processingsection 404, a paper plane position detection section 409, divisionratio determination section 410, image data division section 405,quantization sections 406, 411, quantized data combination section 407and print buffers 408, 412, a mask processing section 413 which areshown in FIGS. 5A and 5B, are formed by the control unit 3000 shown inFIG. 3. Among the above sections, the quantization section 406 and 411correspond to first and second quantization units respectively. Aprocess flow from the input of RGB input image data to the generation ofbinary data for two passes will be explained below with reference toFIGS. 5A and 5B.

The multi-valued image data input section 401 receives an input of RGBimage data that was obtained from an external device such as a digitalcamera 3012 or PC 3010. The color conversion processing section 402converts this RGB image data to devise RGB image data that depends onthe color reproduction range of the printer. The color separationprocessing section 403 converts the device RGB image data tomulti-valued (in this example, 256 values) image data that correspondsto the ink colors used in the printer. The printer of this embodimentuses the four colors of ink of cyan (C), magenta (M), yellow (Y) andblack (K). Therefore, the device RGB image data (R′G′B′) is converted tomulti-valued data (C1, M1, Y1, K1) that corresponds to C, M, Y and Kinks. The color separation processing section 403 uses athree-dimensional look-up-table (LUT) that defines a relation betweeneach of the input values (R′G′B′ input values) of the device RGB imagedata and each of the output values (C1, M1, Y1, K1) of the multi-valuedimage data that corresponds to the ink colors. Here, input values thatare outside the table grid values are calculated by interpolation foroutput values of the surrounding table grid points.

Next, the gradation correction processing section 404 performs thegradation correction process, and the processing after this gradationcorrection processing is performed the same for the colors CMYK.Therefore, in the following explanation, data processing will beexplained for the color black (K) as a representative. The multi-valueddata K1 that is generated by the color separation section 403 is inputto the gradation correction processing section 404. The gradationcorrection processing section 904 performs gradation correction of thismulti-valued data K1 and generates gradation corrected multi-valued dataK2.

A paper plane position detection section (printing position detectionsection) 409 detects the printing position on the printing medium asexplained in FIG. 4. This embodiment, depending on this detectionresult, first, in order to generate two-pass printing data, determineswhether or not to divide and then quantize the multi-value image data,or to quantize the data without dividing the data, then afterwardsdivide that quantized data. Hereafter, the former process will be calleda first process, and the latter process will be called a second process.Moreover, when dividing data, determination is performed of whether ornot to generate common divided data to reflect on the quantized data ofeach pass according to the detected printing position.

More specifically, when the paper plane position detection section 409detects that the printing position is the “middle portion”, thequantization section 411 quantizes the multi-valued data that isobtained by the gradation correction unit 404 as is. In this embodiment,quantization is binarization from which binary data is obtained. Awell-known method such as the error-diffusion method can be used as thisquantization method. In addition, the binarized data is temporarilystored in a buffer 421. Then, a mask processing section 413 performs amask process on the stored binary data, and divides the data intofirst-scan binary data and second-scan binary data for two-passprinting. Furthermore, the divided binary data is read according to thescanning timing of the printing head 5004.

On the other hand, when the paper plane position detection section 409detects one of the “boundary section”, “leading end portion” or“trailing end portion”, the image data division section 405 performsdivision of the multi-valued data as will be described later. At thisdivision process, the division ratio determination section 410determines the division ratios according to the printing position of“boundary section”, “leading end portion” or “trailing end portion”.

More specifically, when the printing position is “boundary”, the imagedata division section 405 performs division at the division ratios 25%,25% and 25% to generate first-scan multi-valued data, second-scanmulti-valued data and first-scan and second-scan common multi-valueddata, respectively, in order to complete two-pass printing. In addition,when the printing position is the “leading end portion” or “trailing endportion” that corresponds to a position near a “boundary”, the imagedata division section 905 performs division at the division ratios 50%,50% and 0% to generate first-scan multi-valued data, second-scanmulti-valued data and first-scan and second-scan common multi-valueddata, respectively. In other words, when the printing position is the“leading end portion” or “trailing end portion” that corresponds to aposition near a “boundary”, the common data is not generated. In thiscase, the quantization section 411 may quantize the multi-valued dataobtained by the gradation correction processing section 404, and by maskprocessing, divide the data into first-scan binary data and second-scanbinary data, as in the case of the “middle portion”.

In this embodiment, the paper plane position detection process that isperformed by the paper plane position detection section 409 is performedafter processing by the gradation correction processing section 909.However, the timing for executing the paper plane position detectionprocessing is not limited to this mode. For example, this processing maybe performed in parallel with processing for the input multi-value imagedata, which is processing from multi-value image data input (401) togradation correction (404).

In the following, the processing by the image data division section 405and later the processing in the case that the paper plane positiondetection section 409 detects the printing position on the printingmedium as a “boundary”, “leading end portion” or “trailing end portion”will be explained in detail.

The image data division section 905 divides the gradation correctedmulti-valued data K2 into first-scan multi-valued data 502 thatcorresponds only to the first scan, second-scan multi-valued data 504that corresponds only to the second scan, and first-scan and second-scancommon multi-valued data 503 that is common to both the first scan andsecond scan. Then, the first-scan multi-valued data 502, first-scan andsecond-scan common multi-valued data 503 and second-scan multi-valueddata 509 are input to the quantization section 406.

The quantization section 406 performs quantization processing (in thisembodiment, binarization processing) for the first-scan multi-valueddata 502, first-scan and second-scan common multi-valued data 503 andsecond-scan multi-valued data 504. Thereby, the first-scan multi-valueddata 502 becomes first-scan quantized data 505, first-scan andsecond-scan common multi-valued data 503 becomes first-scan andsecond-scan common quantized data 506, and second-scan multi-valued data504 becomes second-scan quantized data 507.

In this embodiment, binarization processing employing an exclusiveerror-diffusion method is executed as the quantization processing.Though the exclusive error-diffusion method will be described in detaillater, in short it is a process as follows. The error-diffusionprocessing is performed for the first-scan multi-valued data, thefirst-scan and second-scan common multi-valued data and the second-scanmulti-valued data so that the printing pixels (pixels where dots will beprinted) that are set based on the first through third quantized data(first-scan quantized data as the first quantized data, second-scanquantized data as the second quantized data, and first-scan andsecond-scan common quantized data as the third quantized data) thatcorrespond to three planes become mutually exclusive. That is, thequantization results are controlled so that positions of the printingpixels that are set based on the first-scan quantized data 505, theprinting pixel positions that are set based on the first-scan andsecond-scan common quantized data 506 and the printing pixel positionsthat are set based on the second-scan quantized data 507 do not overlapeach other on the printing medium. Thereby, it is possible to controlthe amount of printing pixels that are set based on the first-scan andsecond-scan common quantized data, or in other words, it is possible tocontrol the amount of pixels for which dots will be printed by both thefirst scan and second scan.

The first-scan quantized data 505, first-scan and second-scan commonquantized data 506 and second-scan quantized data 507 that weregenerated by the quantization section 406 are input to the quantizeddata combination section 407. More specifically, the first-scanquantized data 505 and first-scan and second-scan common quantized data506 are input to a first quantized data combination section 407-1, andthe second-scan quantized data 507 and first-scan and second-scan commonquantized data 506 are input to a second quantized data combinationsection 407-2. By performing a combination process (in this example,logical sum) for the first-scan quantized data 505 and first-scan andsecond-scan common quantized data 506, the first quantized datacombination section 407-1 generates first-scan combined quantized data508. On the other hand, by performing a combination process (in thisexample, logical sum) on the second-scan quantized data 507 andfirst-scan and second-scan common quantized data 506, the secondquantized data combination section 407-2 generates second-scan combinedquantized data 509.

The first-scan combined quantized data 508 and second-scan combinedquantized data 509 that were generated by the quantized data combinationsection 407 are transferred to a print buffer 408. Then, the first-scancombined quantized data 508 is stored in a first-scan buffer 408-1, andthe second-scan combined quantized data 509 is stored in a second-scanbuffer 408-2.

The first-scan combined quantized data that is stored in the first-scanbuffer is read when a first scan is performed and transferred to theprinting head 5004, and dot printing based on the first-scan combinedquantized data is executed during the first scan. Similarly, thesecond-scan combined quantized data that is stored in the second-scanbuffer is read when a second scan is performed and transferred to theprinting head 5004, and dot printing based on the second-scan combinedquantized data is executed during the second scan. Thereby, printing theimage to be printed in the same area can be completed by two scans.

Next, the image data division process (405), quantization process (406)and quantized data combination process (407) described above will beexplained with reference to FIG. 6. FIG. 6 is a diagram illustrating thechange in data values during image processing (image data divisionprocess→quantization process→quantized data combination process). Here,the case of processing multi-valued image data 501 that corresponds to atotal of 24 pixels, 4 pixels (sub scanning direction)×6 pixels (mainscanning direction) will be explained. This multi-valued image data 501corresponds to multi-valued data K2 of the gradation correctionmulti-valued data C2, M2, Y2, K2 that is input to the image datadivision section 405 in FIGS. 5A and 5B.

First, the image data division section 405 divides the multi-valuedimage data 501 into three divisions for each pixel to generatefirst-scan multi-valued data 502, second-scan multi-valued data 504 andfirst-scan and second-scan common multi-valued data 503. In thisdivision, when making the value of the multi-valued image data 501 be A,the value of the first-scan multi-valued data 502 be X, the value of thesecond-scan multi-valued data 504 be Y and the value of the first-scanand second-scan common multi-valued data 503 be Z, division processingis performed so that X+Y+2Z=A is satisfied and that X and Y are nearlythe same value. In order to do this, in this embodiment, divisionprocessing is performed so that the XYZ values become approximately ¼(25%) the value of the multi-valued image data A. More specifically, thequotient α and remainder β (0 to 3) when dividing A by 4 are found, andXYZ values are set based on the quotient α and remainder β as below.

When β=0→X=Y=Z=α

When β=1→X−1=Y=Z=α

When β=2→X−1=Y−1=Z=α

When β=3→X−1=Y=Z−1=α

The values for X, Y and Z that are set in this way respectively becomethe value of the first-scan multi-valued data 502, the value of thesecond-scan multi-valued data 509 and the value of the first-scan andsecond-scan common multi-valued data 503 shown in FIG. 6, respectively.For example, when the value of multi-valued image data A is “160”, α=90and β=0, so X=Y=Z=α=40. It should be noted that the multi-valued imagedata 501 is 256-value data, and the value of A is any of values 0 to255.

Next, the quantization section 406 performs exclusive error-diffusionprocessing for the first-scan multi-valued data 502, the first-scan andsecond-scan common multi-valued data 503 and the second-scanmulti-valued data 509. The threshold value that is used in the errordiffusion processing is “128”. In addition, the Floyd error-diffusionmatrix shown in FIG. 7A is used as the error-distribution matrix thatgives error-distribution coefficients for the peripheral pixels whenperforming error-diffusion processing. The first-scan quantized data 505is binary data that is obtained by quantizing the first-scanmulti-valued data 502, where a “1” denotes a pixel where a dot isprinted, and “0” denotes a pixel where a dot is not printed. Similarly,first-scan and second-scan common quantized data 506 is binary data thatis obtained by quantizing the first-scan and second-scan commonmulti-valued data 503, and second-scan quantized data 507 is binary datathat is obtained by quantizing the second-scan multi-valued data 504. Asis clear from FIG. 6, the positions of the printing pixels that aredefined by these binary quantized data 505 to 507 are such that they donot overlap each other. In this manner, in this embodiment,error-diffusion processing is performed for the multi-valued data 502 to504 of the three planes so that the positions of the printing pixelsthat are defined by the binary quantized data 505 to 507 are eachexclusive. The exclusive error-diffusion processing will be explainedbelow with reference to FIG. 8.

FIG. 8 is a flowchart for explaining the exclusive error-diffusionprocessing. First, the reference symbols in the figure will beexplained. The symbols X, Y and Z, as explained above, denote values ofmulti-valued data (502, 504, 503) for three planes that are input to thequantization section 406 and have a value between 0 and 255. SymbolsXerr, Yerr and Zerr denote accumulated error values that are generatedfrom peripheral pixels for which quantization (binarization) has alreadybeen completed. In this example, in order to conserve the errorsgenerated by quantization processing for each plane, the error of thequantization process generated for each plane is distributed toperipheral pixels within the respective plane. Symbols Xt, Yt and Ztdenote total values of the multi-valued data values (X, Y, Z) and theaccumulated error values (Xerr, Yerr, Zerr). Symbols X′, Y′ and Z′denote the values of quantized data (505, 507, 506) which are theresults of quantization processing (binarization). Symbols X′err, Y′errand Z′err denote error values that occur in the quantization for anobject pixel.

When this processing begins, first, in step S1, the values Xt, Yt and Ztare calculated for the object pixel. Then, in step S2, the values Xt, Ytand Zt are added to each other and it is determined whether or not theadded value (Xt+Yt+Zt) is equal to or greater than the threshold value(128). When the added value is determined to be less than the thresholdvalue, processing advances to step S3, and in order that no printing isperformed for the object pixel by any scan, the binarization results X′,Y′ and Z′ are set to X′=Z′=0. In addition, the errors generated by thisbinarization process are conserved as X′err=Xt, Y′err=Yt and Z′err=Zt,after which processing advances to step S10.

On the other hand, in step S2, when it is determined that the addedvalue is equal to or greater than the threshold value, processingadvances to step S4, and in order to determine the plane for setting theobject pixel as a printing pixel, one maximum value parameter isspecified among the values Xt, Yt and Zt. However, when there are two ormore maximum value parameters, one parameter is specified with the orderof priority being the order Zt, Xt and Yt. The order of priority is notlimited to this, and Xt or Yt could also be set as being the firstpriority.

Next, in step S5, it is determined whether or not the parameterspecified in step S4 is Xt. When it is determined that the specifiedparameter is Xt, processing advances to step S6, and in order that theobject pixel is printed during only the first scan, the binarizationresults X′, Y′ and Z′ are set as X′=1, Y′=0 and Z′=0. Moreover, theerrors that are generated by this binarization processing are conservedas X′err=Xt−255, Y′err=Yt and Z′err=Zt, after which processing advancesto step S10. On the other hand, in step S5, when it is determined thatthe specified parameter is not Xt, processing advances to step S7, andit is determined whether or not the parameter specified in step S4 isYt. When it is determined that the specified parameter is Yt, processingadvances to step S8, and in order that the object pixel is printed inonly the second scan, the binarization results X′, y′ and Z′ are set toX′=0, Y′=1 and Z′=0. Moreover, the errors generated by this binarizationprocessing are conserved as X′err=Xt, Y′err=Yt−255 and Z′err=Zt, afterwhich processing advances to step S10. In step S7, when it is determinedthat the specified parameter is not Yt, processing advances to step S9,and in order that the object pixel is printed in both the first scan andsecond scan, the binarization results X′, Y′ and Z′ are set to X′=0,Y′=0 and Z′=1. Moreover, the errors that are generated by thisbinarization processing are conserved as X′err=Xt, Y′err=Yt andZ′err=Zt−255, after which processing advances to step S10.

In step S10, X′err, Y′err and Z′err that were conserved in step S3, S6,S8 or S9 are distributed to the peripheral pixels in the respectiveplane according to the error-distribution matrix in FIG. 7A. Thequantization processing of the object pixel is completed in this manner,and processing advances to step S11. In step S11, it is determinedwhether or not quantization processing has been completed for all of thepixels, and when processing has not been completed for all pixels,processing returns to step S1, and the similar processing as describedabove is performed for the next object pixel. When processing has beencompleted for all pixels, the exclusive error-diffusion process ends.The accumulated error values (for example, Xerr) that are used in stepS1 are accumulated values of the quantization errors (for example,X′err) that are distributed from one or a plurality of pixels in stepS10.

Through the exclusive error-diffusion processing described above,quantized data (first-scan quantized data 505 (X′), first-scan andsecond-scan common quantized data 506 (Z′) and second-scan quantizeddata 507 (Y′)) are generated as illustrated in FIG. 6 so that thepositions of printing pixels do not overlap each other. In other words,it is possible to control the amount of “1s” (printing dots) in thefirst-scan and second-scan common quantized data that is generatedexclusively from other data, through the division ratio described above.Thereby, as will be described next, it becomes possible to control theamount of overlapping dots that will be printed according to thequantized data for the first scan and second scan that are finallygenerated by combining the common data.

FIG. 6 will be referenced again. The first quantized data combinationsection 407-1 combines the first-scan quantized data 505 and first-scanand second-scan common quantized data 506 using a combination process(in this embodiment, logical sum) to generate binary first-scan combinedquantized data 508. In this first-scan combined quantized data 508,pixels to which a “1” is attached are pixels that become the object ofprinting in the first scan, and pixels to which a “0” is attached arepixels that do not become the object of printing in the first scan.Further, pixels denoted by diagonal lines are pixels that become theobject of printing in both the first scan and second scan. Similarly,the second quantized data combination section 407-2 combines thesecond-scan quantized data 507 and first-scan and second-scan commonquantized data 506 using a combination process (in this embodiment,logical sum) to generate binary second-scan combined quantized data 509.In this second-scan combined quantized data 509, pixels to which a “1”is attached are pixels that become the object of printing in the secondscan, and pixels to which a “0” is attached are pixels that do notbecome the object of printing in the second scan. In addition, pixelsdenoted by diagonal lines are pixels that become the object of printingin both the first scan and second scan in common with each other.

With this embodiment, as described above, it is possible to generatepixels for which dots will be printed commonly in a plurality of scans,and therefore it is possible to suppress fluctuation of the dot coverage(image density fluctuation) that is caused by conveyance error of theprinting medium, movement error of the carriage or the like. Moreover,by quantizing multi-valued data that corresponds in common with aplurality of scan, it is possible to control the amount of pixels forwhich dots are printed in each of the plurality of scans (overlappingdots), and thus it is possible to suppress graininess due to anexcessive amount of overlapping dots from becoming worse. Thereby, it ispossible to keep graininess low level while at the same time suppressdensity fluctuation in an image.

Furthermore, with this embodiment, the first process, which is a processof dividing the multi-valued data and then performing quantization, orthe second process, which is a process of performing quantizationwithout performing division first, and then performing division later,is selected according to the printing position on the printing medium.As a result, for printing positions where it is not so necessary tosuppress density fluctuation and keep graininess low, it is possible tonot perform processing to divide the multi-valued data, and thus it ispossible to suppress an increase in unnecessary processing load.

It should be noted that in this embodiment, division processing isperformed so that the relationship X+Y+2Z A is satisfied and such thatX, Y and Z are nearly the same value to each other. However, theinvention is not limited to this condition. By satisfying therelationship X+Y+2Z=A, the values of the multi-valued data before andafter division do not change, and therefore the density conservation ofthe image is very excellent. However, even though the relationshipX+Y+2Z=A is not satisfied, as long as the value of X+Y+2Z is roughly A,it is possible to sufficiently maintain conservation of the imagedensity. In addition, when executing the processing of this embodiment,pixels occur as illustrated in FIG. 13A for which dots are not printedeven though the value of the multi-valued image data 501 is a value thatindicates the maximum density value (255). In the case of dotarrangement as illustrated in FIG. 11A, the image density becomes lowwhen compared with the dot arrangement of a 100% solid image asillustrated in FIG. 13B. The image density for a dot arrangement asillustrated in FIG. 13A is sufficient. However, when it is desired toexpress a higher density, it is possible to set the values of X, Y and Zso that the total value X+Y+2Z is a value equal to or greater than A.

Embodiment 2

In a second embodiment of the present invention, the division ratios forwhen the printing position is the “leading end portion” and “trailingend portion” differ from those of the first embodiment. Morespecifically, the second embodiment of the invention relates to a modein which the division ratios are made to differ according to thedistance between the “leading end portion” or “trailing end portion”,which is near the “boundary” described above, and the “boundary”.

In this embodiment, the paper plane position detection section 409illustrated in FIG. 5A detects the printing position according to thedistance mentioned above. Then, the division ratio determination section410, as will be explained below, determines the division ratioscorresponding to the aforementioned distance.

FIG. 9 is a graph showing the relationship between the approximation,which is expressed as a ratio of the distance between the position inthe conveying direction of the “leading end portion” or “trailing endportion” and the “boundary” with respect to the overall length in theconveying direction of the “leading end portion” or “trailing endportion”, and the division ratios.

As shown in FIG. 9, at a position of the approximation 100% where theposition is the “boundary”, the first-scan multi-valued data,second-scan multi-valued data and first-scan and second-scan commonmulti-valued data are divided at a percentage of 25% each. On the otherhand, at a position the approximation 0% where the position is the edgeof the leading end portion or trailing end portion, the first-scanmulti-valued data and second-scan multi-valued data are both divided ata percentage of 50% each, and the first-scan and second-scan commonmulti-valued data is divided at a percentage of 0%. In addition, at aposition between an approximation of 100% and 0%, division is performedat division ratios that change linearly.

As described above, in the first embodiment, the change in divisionratios between the “boundary” and “leading end portion” or “trailing endportion” is comparatively large; however, in this embodiment, thedivision ratios can be changed gradually in a linear manner. Morespecifically, in the first embodiment, the common data is used in the“boundary”; however, the common data is not used in the “leading endportion” or “trailing end portion” that are separated from that printingposition. In this case, the dot overlap rate changes comparativelysharply between these areas. On the other hand, with this embodiment, itis possible to change the dot overlap ratio gradually.

When the approximation is 0%, the division ratios are not limited tothose described above. For example, as explained in the firstembodiment, quantization may be performed without dividing the gradationcorrected multi-valued data, after which the quantized data can bedivided using a mask process.

Other Embodiments

Printing positions on a printing medium in the embodiments explainedabove are separated according to whether or not the printing medium isheld by respective conveying roller pairs provided on upstream anddownstream sides of the printing head. However, of course application ofthe present invention is not limited to this form. It may be possible todistinguish printing positions by the construction for conveyingprinting medium in the printer. That is, for generating printing datafor a printing position in a conveyance condition in which comparativelylarge conveyance error occurs during conveyance of the printing medium,it is possible to select a process as described above according to theoccurring conveyance error such as selecting the process described abovein which the multi-valued data is divided and then quantized.

Further Embodiments

In each of the embodiments described above, when dividing multi-valueddata, error-diffusions are performed exclusively for three multi-valueddata. However, the error-diffusion does not have to be exclusive. Morespecifically, error-diffusion processing may be performed for the threemulti-valued data using the three types of error-distribution matricesshown in FIGS. 7A to 7C.

To the quantization section 406 shown in FIG. 5A, the first-scanmulti-valued data 502, first-scan and second-scan common multi-valueddata 503 and second-scan multi-valued data 504 that are generated by theimage data division section 405 are input. The quantization section 406generates first-scan quantized data 505 by performing binaryerror-diffusion processing for the first-scan multi-valued data 502. Inthis generation, “128” is used as a threshold value (predeterminedvalue), and the error-distribution matrix shown in FIG. 7B is used asthe error-distribution matrix. Moreover, the quantization section 406generates first-scan and second-scan common quantized data 506 byperforming binary error-diffusion processing for the first-scan andsecond-scan common multi-valued data 503. In this generation, “128” isused as a threshold value, and the error-distribution matrix shown inFIG. 7A is used as the error-distribution matrix. Furthermore, thequantization section 406 generates second-scan quantized data 507 byperforming binary error-diffusion processing for the second-scanmulti-valued data 509. In this generation, “128” is used as a thresholdvalue, and the error-distribution matrix shown in FIG. 7C is used as theerror-distribution matrix.

By using different error-distribution matrices among three planes inthis manner, the quantization results for the three planes (positions ofprinting pixels that are set by the quantized data 505 to 507) can bemade different. Thereby, pixels to be printed only by the first scan andpixels to be printed only by the second scan can be generated whilepixels (overlapping dots) to be printed by both the first scan andsecond scan are generated. If the same error-distribution matrix is usedamong three planes, the quantization results for the three planes arevery similar. In that case, the pixels that are printed by the firstscan and the pixels that are printed by the second scan are nearly thesame, and even when printing an image with the maximum density, ofalmost all of the printing pixels half are overlapping dots, and for theother half of the pixels, blank pixels where no dots are printedincrease. In such a case, it is difficult to conserve the input valuesin the density of the output image. However, in this embodiment, thequantization results for the three planes differ, and not only thepixels that are printed by both the first scan and second scan, but alsothe pixels that are printed only in the first scan and the pixels thatare printed only in the second scan are generated. Therefore, it ispossible to maintain the output image density to a certain extent.

In addition, the respective positions of the printing pixels (pixels towhich “1” is assigned) defined by the binary quantized data 505 to 507that are generated by the processing of this embodiment are not in aperfectly exclusive relationship with each other. Accordingly, there isa possibility that the respective positions of printing pixels willoverlap. For example, there may be cases in which the binarizationresult for a certain pixel is “1” for both the quantized data 505 andquantized data 506. Therefore, by applying a logical sum process as thecombination process after that as in the first embodiment, the number ofprinting pixels after the combination process becomes less than thenumber of printing pixels that is defined by the quantized data 505 to507. In that case, the ability to conserve the input values in thedensity of the output image decreases. When this decrease in the densitycan be allowed, the logical sum process can be applied as thecombination process. However, when such a decrease in density cannot beallowed, a combination process can be executed in which the values ofthe quantized data (“1” or “0”) are added for each pixel, such that theadded value becomes the value of the combined quantized data. Forexample, for a certain pixel A, when the value of both the quantizeddata 505 and the quantized data 506 is “1”, the value of the first-scancombined quantized data 508 is taken to be 2 (=1+1). Then the number ofdots that corresponds to the added value (0, 1, 2) is printed in eachscan. Thereby, printing can be performed without a decrease in thepreservation of the input values in the density of the output image.

With the embodiment described above, as explained in the firstembodiment, it is possible to control the amount of pixels (overlappingdots) that are printed in both the first scan and second scan, so thatit is possible to suppress both image density fluctuation and aworsening of graininess as described above. In addition, with thisembodiment, error-diffusion processing is performed independently forthree planes, so when compared with performing exclusive error-diffusionprocessing as in the first embodiment, it is possible to improve theprocessing speed.

In this embodiment, in order to make the quantization results for threeplanes different, the case is explained in which the error-distributionmatrices that are used for the planes are different. However, theembodiment is not limited to this. For example, instead of the aboveconfiguration, the error-distribution matrices that are used among theplanes may be the same, and the threshold values that are used for theplanes may be different. Moreover, it is possible to make a combinationof error-distribution matrices and threshold values different for theplanes.

Furthermore, as another embodiment, instead of the quantization section406 performing quantization by the error-diffusion method as in thefirst and second embodiments, it may be possible for the quantizationsection 406 to perform quantization by using a dither method.

In this case, to the quantization section 406 shown in FIG. 5A, thefirst-scan multi-valued data 502, first-scan and second scan commonmulti-valued data 503 and second-scan multi-valued data 504 that aregenerated by the image data division section 405 are input. Thequantization section 406 performs dither processes for the first-scanmulti-valued data 502, first-scan and second scan common multi-valueddata 503 and second-scan multi-valued data 504 using different dithermatrices for each. By performing the dither processes (quantizationprocesses) using three different dither matrices in this manner, it ispossible to generate three different quantized data 505 to 507 as thequantization result.

With this embodiment, it is possible to suppress both image densityfluctuation and the worsening of graininess while at the same timemaintain to a certain extent the conservation of output image density asin the embodiments described above. In addition to this, in thisembodiment, the dither process is performed independently formulti-valued data of three planes, and accordingly it is possible tofurther improve the processing speed. Furthermore, in this embodiment,the dither process is performed using three different dither matrices,and accordingly control of the dot arrangement in each scan and spatialfrequency of the overlapping dot arrangement among scans becomes easierthan in the case of performing error-diffusion processing.

In the embodiments described above, when dividing the multi-valued data,binarization processing is performed as the quantization processing.However, as yet another embodiment, three-value quantization processingmay be performed as the quantization processing. Except for this pointthe embodiment is the same as the other embodiments described above. Inthis embodiment, the binarization processing in all of the embodimentsdescribed above can be replaced with three-value quantizationprocessing. However, here the case of replacing the binarizationprocessing of the first embodiment with three-value quantizationprocessing will be explained. In this embodiment, three-value exclusiveerror-diffusion processing is performed on the multi-valued data 502 to504 so that the positions of printing pixels that are defined by each ofthe quantized data 505 to 507, which are quantized to three valued data,do not overlap each other.

FIGS. 10A and 10B are flowcharts for explaining the three-valueexclusive error-diffusion processing. The meaning of the symbols (Xt,Xerr, X′ and the like) in FIGS. 10A and 10B are the same as the meaningof the symbols in FIG. 8. In this embodiment, as threshold values, afirst addition threshold value (170) and a second addition thresholdvalue (85) are used. Moreover, the values of the three-valuequantization results X′, Y′ and Z′ are any one of “0”, “1” or “2”. Here,“0” indicates that no dot is printed, “1” indicates that one dot isprinted, and “2” indicates that two dots are printed.

When this processing begins, first in step S1, the values Xt, Yt and Ztare calculated for the object pixel. Next, in step S2, the added valueAt that is obtained by adding Xt, Yt and Zt (=Xt+Yt+Zt) is obtained.Then in step S3, it is determined whether the added value At is equal toor greater than the first addition threshold value (170), whether theadded value At is less than the first addition but equal to or greaterthan the second addition threshold value (85), and whether the addedvalue At is less than the second addition threshold value.

In step S3, when it is determined that the added value At is less thanthe second threshold value (85), processing advances to step S16, and inorder that the object pixel is not printed in any of the scans, theresults of the three-value quantization are set to X′=Y′=Z′=0. Moreover,the errors that are generated by this three-value quantization processare saved as X′err=Xt, Y′err=Yt and Z′err=Zt, and then processingadvances to step S17.

On the other hand, in step S3, when the added value At is determined tobe equal to or greater than the first addition threshold value (170),processing advances to step S4, and in order to determine the plane forsetting the object pixel to a printing pixel (“1”), one of theparameters having the maximum value is identified from Xt, Yt and Zt.However, when there are two or more maximum value parameters, oneparameter is identified in the order of priority Zt, Xt and Yt. Theorder of priority is not limited to this, and Xt or Yt could be thefirst priority. Next, in step S5, it is determined whether or not themaximum value parameter identified in step S4 is Xt. When the parameteris determined to be Xt, processing advances to step S6, and in orderthat two dot are printed for the object pixel in the first scan, thethree-value quantization results are set as X′=2, Y′=0 and Z′=0.Moreover, the errors that are generated by this three-value quantizationprocess are saved as X′err=Xt−255, Y′err=Yt and Z′err=Zt, after whichprocessing advances to step S17. On the other hand, in step S5, when itis determined that the parameter is not Xt, processing advances to stepS7, and it is determined whether or not the maximum value parameter thatis identified in step S4 is Yt. When it is determined that the parameteris Yt, processing advances to step S8, and in order that two dots areprinted for the object pixel in the second scan, the three-valuequantization results are set as X′=0, Y′=2 and Z′=0. Moreover, theerrors that are generated in this three-value quantization process aresaved as X′err=Xt, Y′err=Yt−255 and Z′err=Zt, after which processingadvances to step S17. In step S7, when it is determined that theparameter is not Yt, processing advances to step S9, and in order thattwo dots are printed for the object pixel in both the first scan andsecond scan, the binarization results are set as X′=0, Y′=0 and Z′=2.Moreover, the errors that are generated in this binarization process aresave as X′err=Xt, Y′err=Yt and Z′err=Zt−255, after which processingadvances to step S17.

On the other hand, in step S3, when it is determined that the addedvalue At is less than the first addition threshold value (170) and equalto or greater than the second addition threshold value (85), processingadvances to step S10. In step S10, in order to determine the plane forsetting the object pixel to the printing pixel (“2”), one maximum valueparameter is identified from Xt, Yt and Zt according to the same rulesas in step S4. Next, in step S11, it is determined whether or not themaximum value parameter that is identified in step S10 is Xt. When it isdetermined that the parameter is Xt, processing advances to step S12,and in order that one dot is printed for the object pixel in the firstscan, the three-value quantization results are set to X′=1, Y′=0 andZ′=0. Moreover, the errors that are generated by this three-valuequantization process are saved as X′err=Xt−128, Y′err=Yt and Z′err=Zt,after which processing advances to step S17. On the other hand, in stepS11, when it is determined that the parameter is not Xt, processingadvances to step S13 and it is determined whether or not the maximumparameter identified in step S10 is Yt. When it is determined that theparameter is Yt, processing advances to step S14, and in order that onedot is printed for the object pixel in the second scan, the three-valuequantization results are set to X′=0, Y′=1 and Z′=0. Moreover, theerrors that are generated by this three-value quantization process aresaved as X′err=Xt, Y′err=Yt−128 and Z′err=Zt, after which processingadvances to step S17. In step S13, when it is determined that theparameter is not Yt, processing advances to step S15, and in order thatone dot each is printed for the object pixel in both the first scan andsecond scan, the binarization results are set to X′=0, Y′=0 and Z′=1.Moreover, the errors that are generated by this binarization processingare saved as X′err=Xt, Y′err=Yt and Z′err=Zt−128, after which processingadvances to step S17.

In step S17, the errors X′err, Y′err and Z′err that are saved in stepS6, S8, S9, S12, S14, S15 or S16 are distributed to the peripheralpixels of its own plane according to the error-distribution matrix shownin FIG. 7A. In this way, quantization processing of an object pixel endand processing advances to step S18. In step S18, it is determinedwhether or not quantization processing has ended for all of the pixels,and when processing has not yet ended for all of the pixels, processingreturns to step S1 and the similar processing is performed for the nextobject pixel, and when processing has ended for all of the pixels, theexclusive error-diffusion processing ends. The exclusive error-diffusionprocessing described above allows quantized data (first-scan quantizeddata 505 (X′), first-scan and second-scan common quantized data 506 (Y′)and second-scan quantized data 507 (Z′)) to be generated for threeplanes, so that the positions of the printing pixels do not overlap eachother.

With the embodiment described above, in addition to the effect obtainedby the other embodiments, it is possible to obtain an image having evenbetter gradation expression than the embodiments above in whichbinarization processing is performed. When a plurality of dots areformed in a pixel area, ink may be ejected a plurality of times towardthe same position in the pixel area, or ink may be ejected a pluralityof times toward different positions in the pixel area.

This embodiment is not limited to the case in which binarizationprocessing that is explained in the first embodiment is replaced withthree-value quantization processing, and it is possible to replace thebinarization processing of the embodiments that use the three kinds oferror-diffusion matrices described above or the embodiments that use thedither method with three-value quantization processing. Whenbinarization is replaced with three-value quantization in this way, itis possible to execute three-value error-diffusion processing orthree-value dither as the quantization process. In this case, in orderto make the three-value quantization results different among planes,using different error-distribution matrices or different dither matricesfor each plane is the same as in the embodiments described above. Inthis manner, the quantization section 906 generates three-valuequantized data 505 to 507 having different quantization results. Thepositions of the printing pixels that are defined by these three-valuequantized data do not have a completely exclusive relationship with eachother, and accordingly there may be cases in which the positions of theprinting pixels overlap. Therefore, it is preferred that a combinationprocess in which the quantized values are added for each pixel as in theembodiments described above be applied as the following combinationprocess.

In the embodiments described above, two-pass printing in which a imageto be printed in the same area is completed by two scans is explained.However, as another embodiment it is possible to apply multi-passprinting of three passes or more. In the following, as an example ofmulti-pass printing of three passes or more an example of the case ofthree-pass printing will be explained. Features of this embodiment arean image data division process, quantization process and quantized datacombination process, and except for these features, processing is thesame as in the embodiments described above. Only the image data divisionprocess, quantization process and quantized data combination processwill be explained below with reference to FIG. 11.

FIG. 11 is a diagram schematically illustrates the flow of imageprocessing that is executed by the image data division section 405,quantization section 406 and quantized data combination section 407shown in FIGS. 5A and 5B (image data division process→quantizationprocess→quantized data combination process). As is explained in thefirst embodiment, gradation corrected multi-valued data K2 (multi-valuedimage data 501) is input to the image data division section 405.

The image data division section 405 divides the input multi-valued imagedata into first-scan multi-valued data 901 that corresponds only to thefirst scan, second-scan multi-valued data 902 that corresponds only tothe second scan, third-scan multi-valued data 903 that corresponds onlyto the third scan, first-scan and second-scan common multi-valued data904 that corresponds to both the first scan and second scan, first-scanand third-scan common multi-valued data 905 that corresponds to both thefirst scan and third scan, second-scan and third-scan multi-valued data906 that corresponds to both the second scan and third scan, andfirst-scan, second-scan and third-scan common multi-valued data 907 thatcorresponds to all of the first scan, second scan and third scan.

Next, the quantization section 406 performs binary exclusiveerror-diffusion as explained for the first embodiment on these sevenplanes of multi-valued data 901 to 907. Thereby, first-scan quantizeddata 911, second-scan quantized data 912, third-scan quantized data 913,first-scan and second-scan common quantized data 914, first-scan andthird-scan common quantized data 915, second-scan and third-scan commonquantized data 916, and first-scan, second-scan and third-scan commonquantized data 917 are generated.

Next, these seven planes of quantized data 911 to 917 are input to thequantized data combination section 907 and the quantized data 911 to 917are combined for each corresponding scan. More specifically, first-scanquantized data 911, first-scan and second-scan common quantized data914, first-scan and third-scan common quantized data 915 and first-scan,second-scan and third-scan common quantized data 917 are input to afirst quantized data combination section 407-1. The first quantized datacombination section 407-1 combines the quantized data 911, 914, 915 and917 (logical sum in this embodiment) and generates first-scan combinedquantized data 921. Moreover, second-scan quantized data 912, first-scanand second-scan common quantized data 914, second-scan and third-scancommon quantized data 916 and first-scan, second-scan and third-scancommon quantized data 917 are input to a second quantized datacombination section 407-2. The second quantized data combination section407-2 combines the quantized data 912, 914, 916 and 917 and generatessecond-scan combined quantized data 922. Furthermore, third-scanquantized data 913, first-scan and third-scan common quantized data 915,second-scan and third-scan common quantized data 916 and first-scan,second-scan and third-scan common quantized data 917 are input to athird quantized data combination section 407-3. The third quantized datacombination section 407-3 combines the quantized data 913, 915, 916 and917 and generates third-scan combined quantized data 923. By performingthe processing described above it is possible to generate printing datafor three passes. With this embodiment, the effect obtained in the firstembodiment can be achieved in multi-pass printing of three passes ormore. In this way quantized data are combined for each relative scan ofthe printing head over the printing medium.

In this embodiment, exclusive error-diffusion as explained in the firstembodiment is applied as the quantization process. However, thequantization process that can be applied to this embodiment is notlimited to this method. For example, it may be possible to apply anindependent error-diffusion process or independent dither process asexplained in the embodiment that use three kinds of error-diffusionmatrices or embodiment that uses a dither method. Moreover, thequantization process that can be applied to this embodiment is notlimited to binarization processing, and three-value or four-value orgreater quantization processing as was explained in the other embodimentcan be applied.

Moreover, in this embodiment, division processing was performed so thatcommon multi-valued data is generated for all combinations of first,second and third scans; however, the division processing that can beapplied in this embodiment is not limited to this. For example, it ispossible to generate common multi-valued data so that overlapping dotsare generated between certain scans (first scan and second scan). Inthat case, in addition to first-scan multi-valued data 901, second-scanmulti-valued data 902 and third-scan multi-valued data 903, it ispossible to generate only first-scan and second-scan common multi-valueddata 904 as common multi-valued data, and not generate first-scan andthird-scan common multi-valued data 905, second-scan and third-scancommon multi-valued data 906 or first-scan, second-scan and third-scancommon multi-valued data 907.

The technical idea of the present invention is the creation of pixels inwhich dots are printed in at least two scans, and accordingly,regardless of the number of multi passes, by generating correspondingmulti-valued data that is common for at least two scans, it is possibleto obtain the effect of the present invention. Therefore, in the presentinvention, when performing printing by M (M is an integer 2 or greater)scans, in addition to multi-valued data that corresponds to M scans,corresponding multi-valued data that is common for at least two scansmay be generated and it is not necessary to generate multi-valued datathat is common for all M scans.

Furthermore, forms that are obtained by suitably combining theembodiments described above as another embodiment are included withinthe range of the present invention.

In the embodiments described above, the case of using a serial-typeprinter that performs multi-pass printing by ejecting in from a printinghead while the printing head moves with respect to a printing medium(relative scan) is explained. However a printing to which the presentinvention is applied is not limited to this type of printing apparatus.The present invention may be applied to a full-line type printingapparatus that performs multi-pass printing by ejecting ink while aprinting medium is conveyed with respect to a printing head (relativescan). For example, a system is possible in which there is one printinghead for one ink color, and a plurality of relative scans is performedby moving the printing head back and forth with respect to a printingmedium. That is, the present invention may be applied as long asmulti-pass printing is performed during relative scan between a printinghead and printing medium.

Moreover, examples of binarization or three-value quantization wereexplained for the embodiments described above. However, the quantizationprocessing that can be applied to the present invention is not limitedto this, and 4-value or greater quantization processing can be applied.In other words, in the present invention, N-value (N is an integer 2 orgreater) quantization can be applied. Therefore, a form in which theembodiments described above are changed to N-value quantizationprocessing is included in the range of the present invention.

Furthermore, in the embodiments described above, the quantizationsection 406 performs quantization of a plurality of planes ofmulti-valued data at the same time (in parallel). However, applicationof the present invention is not limited to this form, and it is possibleto sequentially perform quantization of a plurality of planes ofmulti-valued data. FIG. 12 is a block diagram that illustrates the stepsof this process. In FIG. 12, first-scan and second-scan commonmulti-valued data quantization (2103-12), first-scan multi-valuequantization (2103-1) and second-scan multi-value quantization (2103-2)are in no particular order, and quantization can be performed in anyorder.

By performing quantization sequentially in this way, it is possible toreduce the amount of memory used during the quantization process whencompared with performing quantization simultaneously (in parallel). Forexample, when performing error-diffusion processing as the quantizationprocess, it is possible to reduce the amount of error storage memoryused for storing error values that are generated in the error-diffusionprocess.

In the embodiments described above, a form in which the four colors ofink CMYK are used was explained. However the number of types of ink thatcan be used is not limited to this. It is possible to add light cyan(Lc) and light magenta (Lm) ink to the for colors of ink above, and itis also possible to add special inks such as red ink (R) or blue ink(B). In addition, in the embodiments described above, the case wasexplained in which a color printing mode is executed that uses aplurality of colors of ink; however, the present invention can also beapplied to a mono color mode in which only a single color of ink isused. Furthermore, the present invention can be applied to not only acolor printer, but also a monochrome printer.

In the embodiments described above, the image processing apparatus thatexecutes the image processing that is a feature of the present inventionis explained using an example of a printer (image formation apparatus)that comprises a control unit 3000 having an image processing function;however, the present invention is not limited to this kind ofconstruction. Construction may be such that a host apparatus (forexample the PC 3010 in FIG. 3) in which a printer driver is installedexecutes the image processing that is a feature of the presentinvention. In that case, the host apparatus that is connected to theprinter corresponds to the image processing apparatus of the presentinvention. It should be noted that when the host apparatus executes theimage processing described in the above embodiments, for example, thehost apparatus detect a position corresponding to the specified printingposition based on input RGB image data or multi-valued image data thatis obtained by the color conversion and perform the first process forimage data corresponding to the detected position to generate data.

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (e.g., computer-readable medium).

while the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-287538, filed Dec. 18, 2009, which is hereby incorporated byreference herein in its entirety.

1. An image processing apparatus that, for printing an image on a pixelarea of a printing medium at respective printing positions on theprinting medium by performing a plurality of relative scans of aprinting head with respect to the printing medium that is conveyed todifferentiate the printing positions, processes multi-valued image datathat corresponds to the image to be printed on the pixel area, saidapparatus comprising: a selecting unit configured to select one of afirst process and a second process that process the multi-valued imagedata corresponding to the printing position in accordance with theprinting position on the printing medium; in the case that saidselecting unit selects the first process, a first division unitconfigured to divide the multi-valued image data into respectivemulti-valued data that correspond to the plurality of relative scans andmulti-valued data that corresponds commonly to at least two relativescans among the plurality of relative scans; a first quantization unitconfigured to perform quantization processing for each of themulti-valued data divided by said first division unit to generaterespective quantized data that correspond to each of the plurality ofrelative scans and quantized data that corresponds commonly to the atleast two relative scans; and a combination unit configured to combinethe quantized data generated by said first quantization unit for eachcorresponding relative scan to generate combined quantized data thatcorresponds to each of the at least two relative scans; or in the casethat said selecting unit selects the second process, a secondquantization unit configured to perform quantization processing for themulti-valued image data to generate quantized data; and a seconddivision unit configured to divide the quantized data generated by saidsecond quantization unit into respective quantized data that correspondto each of the plurality of relative scans.
 2. The image processingapparatus according to claim 1, wherein said selecting unit selects thesecond process in the case that the printing position is a position of amiddle portion when the printing medium is conveyed in a condition thatthe printing medium is held by both an upstream roller pair and adownstream roller pair, the printing position is a position of a leadingend portion when the printing medium is conveyed in a condition that theprinting medium is held only by the upstream roller pair, or theprinting position is a position of a trailing end portion when theprinting medium is conveyed in a condition that the printing medium isheld only by the downstream roller pair, and selects the first processin the case that the printing position is a position of a boundarybetween the leading end portion and the middle portion or is a positionof a boundary between the trailing end portion and the middle portion.3. The image processing apparatus according to claim 1, wherein saidselecting unit selects the second process in the case that the printingposition is a position of a middle portion when the printing medium isconveyed in a condition that the printing medium is held by both anupstream roller pair and a downstream roller pair, and selects the firstprocess in the case that the printing position is a position of aleading end portion when the printing medium is conveyed in a conditionthat the printing medium is held only by the upstream roller pair, theprinting position is a position of a trailing end portion when theprinting medium is conveyed in a condition that the printing medium isheld only by the downstream roller pair, or the printing position is aposition of a boundary between the leading end portion and the middleportion or is a position of a boundary between the trailing end portionand the middle portion.
 4. The image processing apparatus according toclaim 3, wherein said first division unit determines division ratiosused for dividing the multi-valued image data into respectivemulti-valued data that correspond to the plurality of relative scans andmulti-valued data that corresponds commonly to the at least two relativescans, according to a ratio of a distance between the position in theconveying direction of the leading end portion or trailing end portionand the boundary with respect to the overall length in the conveyingdirection of the leading end portion or trailing end portion.
 5. Theimage processing apparatus according to claim 1, further comprising: aprinting unit configured to print an image on the printing medium byusing the printing head; and a detection unit configured to detect theprinting position on the printing medium, in order to form an imageforming apparatus.
 6. An image processing method of, for printing animage on a pixel area of a printing medium at respective printingpositions on the printing medium by performing a plurality of relativescans of a printing head with respect to the printing medium that isconveyed to differentiate the printing positions, processingmulti-valued image data that corresponds to the image to be printed onthe pixel area, said method comprising: a detection step of detectingthe printing position on the printing medium; and a selecting step ofselecting one of a first process and a second process that process themulti-valued image data corresponding to the printing position inaccordance with the printing position detected by said detection step,wherein the first process including: a first division step of dividingthe multi-valued image data into respective multi-valued data thatcorrespond to the plurality of relative scans and multi-valued data thatcorresponds commonly to at least two relative scans among the pluralityof relative scans; a first quantization step of performing quantizationprocessing for each of the multi-valued data divided by said firstdivision unit to generate respective quantized data that correspond toeach of the plurality of relative scans and quantized data thatcorresponds commonly to the at least two relative scans among theplurality of relative scans; and a combination step of combining thequantized data generated by said first quantization unit for eachcorresponding relative scan to generate combined quantized data thatcorresponds to each of the at least two relative scans; and the secondprocess including: a second quantization step of performing quantizationprocessing for the multi-valued image data to generate quantized data;and a second division step of dividing the quantized data generated bysaid second quantization unit into respective quantized data thatcorrespond to each of the plurality of relative scans.
 7. A program thatis read by a computer and makes the computer function as an imageprocessing apparatus according to claim 1.